Structured non-Bt cotton refuges may still be necessary

Elton Robinson | Jun 03, 2004

WINNSBORO, La. – A Louisiana crop consultant and an LSU AgCenter entomologist are concerned that data developed in a recent Bollgard resistance study may be used to de-emphasize the need for a structured refuge strategy for Bt cotton across all states of the Cotton Belt.

For the past two years, the multi-state research project examined the influence of alternate crops on bollworm population dynamics. EPA requested the study to determine whether non-cotton host plants such as corn, soybeans, grain sorghum and certain weeds can harbor corn earworm (cotton bollworm) in sufficient numbers to supplement the 5 percent Bollgard refuge option.

The value of the 5 percent refuge has been questioned as to whether it could produce enough pests to satisfy resistance management efforts. Without this option, the EPA considers only two viable options for resistance management – the 20 percent sprayed refuge option or the 5 percent embedded option.

The study, sponsored by Monsanto and USDA’s Agricultural Research Service, does show that alternate host plants can produce large numbers of cotton bollworm and could supplement the 5 percent option.

That data was also used to suggest that if alternate hosts provide a refuge for susceptible cotton bollworms, then structured refuges might not be necessary in southeastern cotton states, where the bollworm is the primary Heliothine pest.

LSU AgCenter entomologist Roger Leonard noted that in multiple cropping systems that have a strong mix of alternate hosts for bollworm and where cotton does not represent a majority of acreage, the concept “may have considerable merit. The key to the success of alternate hosts will be to provide a consistent source of bollworms in appropriate densities at the same time that bollworm is found in Bt cotton.”

But the situation is different for the tobacco budworm, which was only a secondary consideration in the study.

Previous studies in the Mid-South have shown that “cotton has been the only consistent host contributing large numbers of tobacco budworms to the cotton ecosystem,” Leonard said.

“The tobacco budworm is the one we better be concerned about,” added Ray Young, a cotton producer and consultant from Wisner, La. “We don’t know where the alternate hosts for tobacco budworm are. There’s not many in our part of the country.”

If you took away the structured refuge, there would be no way to protect susceptible tobacco budworms, Leonard points out. “If resistance to Bt develops, it is likely that we will see a total breakdown in insect pest management. If and when this happens, control costs would dramatically increase and cotton acreage would probably decline.”

“We need a lot more documentation than we have now,” Young added. “I don’t think we can rely on just models that says if this happens or that happens, resistance is going to happen in 50 years. We need to know what’s happening in the real world.”

Leonard noted that one reason why the tobacco budworm wasn’t included in the major part of the alternate host study was that Bt cotton is extremely effective against tobacco budworm. In fact, tobacco budworm larvae have never been detected in pure stands of Bt cotton fields, according to Leonard.

This level of effectiveness has a resistance management implication referred to as a “high dose” strategy. Basically, it raises the bar for an insect under a high dose regime to become resistant. But this doesn’t mean that an insect under this regime can’t find a path to resistance.

“Historically, when a high level of selection has been place on an insect species, resistance tended to occur at a relatively rapid pace,” Leonard said. “If we ever see a change in Bt susceptibility for tobacco budworm, I’m afraid it will be too late. It will likely be unmanageable.”

The bollworm is more tolerant to Bt proteins, in general, which is why Bt fields are treated when high densities of bollworms occur. The entomologist noted that in some two-gene Bt products, the high dose strategy may apply to the cotton bollworm.

And if two-gene products dominate cotton acreage with no structured refuges, alternate host plants may be the only source of susceptible insects. Selection pressure in cotton fields would be tremendous, according to Leonard.

“There is no guarantee that simply adding another gene is the proper direction for resistance management without incorporating other resistance management tools. These pests could develop resistance to multiple Bt proteins. They have already had selection pressure with a single protein event. Just adding another protein may or may not compensate for the fact that these insects have been previously exposed.”

The fact that Mid-South cotton producers haven’t had a serious, widespread outbreak of tobacco budworm since Bt cotton is pushing many farmers to think more about the bottom line of maintaining a refuge.

“I believe that refuges cost producers money in the short term, there’s no doubt about it,” Leonard said. “Proper refuge management is difficult for producers because it requires additional planning and consideration by agricultural consultants. The technology companies are required to monitor compliance which costs them money and producer support.”

Farmers don’t have a lot of choices of what to plant in their refuges, either, noted Leonard. “Many of the varieties that are planted in refuges do not perform as well as similar Bt varieties. In some instances, this is simply due to the different insect management tactics required in Bt versus non-Bt cotton. In addition, much of the premier germplasm is going to contain the Bt gene first. ”

Good arguments all, says Leonard. But much could be lost if heliothine pests like tobacco budworm and cotton bollworm become resistant to Bt cotton, including millions of dollars spent on research to develop the technology, and a decade of developing elite Bt cotton varieties.

“If we completely lost the technology due to resistance, we would try to produce cotton with Tracer, Steward or Denim. You’re going to trade roughly $25-30 an acre for costs that could easily exceed $100 an acre.

“We must do whatever is necessary to protect this technology,” Leonard said. “If the data supports a structured refuge strategy and this option is effective in delaying resistance to tobacco budworm, then it has to be done for long-term sustainability of cotton producers. I sincerely hope that the cotton industry is allowed to maintain all resistance management options and as more studies are completed, even more options can be added.”

“This technology is just too valuable to take chances with,” Young added. “The little inconvenience to plant a refuge is certainly well worth the benefits that we will receive in preventing resistance.”

For more information on this an other topics, see the June 4 issue of Delta Farm Press.